1. Field of the Invention
The present invention relates to a fiber type optical wavelength converter device for converting a fundamental wave into a second harmonic wave having a wavelength which is 1/2 of the wavelength of the fundamental wave, and more particularly to an optical wavelength converter device which is capable of converting the wavefront of a wavelength-converted wave from a conical wavefront into a planar or spherical wavefront. The present invention also relates to an optical wavelength converter system which comprises a fiber-type optical wavelength converter device for converting a fundamental wave into a second harmonic wave whose wavelength is 1/2 of the wavelength of the fundamental wave, and an optical device for converting the wavefront of a wavelength-converted wave from a conical wavefront into a planar or spherical wavefront.
2. Description of the Prior Art
Various attempts have heretofore been made to convert a wave of laser beam, which has been applied as a fundamental wave to a nonlinear optical material, to its second harmonic or the like (i.e., to shorten the wavelength of the laser beam). One well known example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk-crystal-type optical wavelength converter device as disclosed, for example, in Introduction to Optical Electronics, pages 200-204, written by A. Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K.K.). This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence cannot be employed even if it has high nonlinearity.
To solve the above problem, there has been proposed a fiber-type optical wavelength converter device. An optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of a nonlinear optical material and surrounded by a cladding. One example of such an optical fiber is shown in Vol. 3, No. 2, pages 28-32, of the Bulletin of the Microoptics Research Group of a gathering of the Applied Physics Society. Recently, much effort has been directed to the study of a fiber-type optical wavelength converter device since it can easily achieve phase matching between a guided mode, in which a fundamental is guided through the core, and a radiated mode, in which a second harmonic is radiated into the cladding (for so-called Cerenkov radiation).
The wavelength-converted wave produced by the fiber-type optical wavelength converter device is emitted from the output end of the cladding and has various applications. In many of the applications, the wavelength-converted wave is converged into a small spot. For example, the wavelength-converted wave is focused into a very small spot for higher recording density, when the wavelength-converted wave is used to record optical signals optically.
However, it is recognized that the wavelength-converted wave emitted from the fiber Cerenkov-type optical wavelength converter device cannot converge into a small spot even if the emitted wave is passed through a general spherical lens.